This invention relates to methods for constructing tunnels and particularly for constructing tunnels between starting shafts and end shafts.
A variety of methods are known for tunnel construction. The methods of tunnels advance and the optimal equipment selection primarily are determined by the intended use, the cross-section and the lining required, as well as by the prevailing rock or soil types. Of utmost importance are the criteria of costs and safety of the personnel driving the tunnel and costs and safety of the tunnel project itself, particularly when encountering difficult terrain.
A conventional method which is used in loose soils and loose rocks employs so-called poling plates or spiles and provides that soil is shored up before excavation is begun. However, tunnel driving with poling plates cannot, or at best, only partially can be employed in soils which have shallow consistencies or in soils which are under water pressure. Moreover, deformations of the poling plates or deviations of the excavating equipment present great difficulties.
The shield method of tunneling is a semi-mechanical method. It is suitable for use in driving tunnels having large cross-sections, both in dry soils and in soils under water, using compressed air. The shield, a large pipe-like construction made of steel, engages the face side of the tunnel with its front cutting edge. Powerful jacks drive the shield forward, cutting the tunnel shape while maintaining the cross section. Breasting of the face is provided as required. A lining is formed immediately behind the shield. Segmental or tubular portions of the lining are made of cast iron, steel or precast concrete and are installed under the protection of the rear portion of the shield, the shield tail. The shield-advancing jacks are supported against the lining. Shields are used in clay, sand and gravel. The shield driving method finds only marginal application if larger hard rock inclusions are encountered.
A variation thereof is compressed air shielding, which is employed in the presence of ground water. This very costly process is applied only if lowering of ground water is not possible. Limits of compressed air shielding, as in normal shield driving, are well defined. Furthermore, tunnels cannot be driven by compressed air shielding in changing soils or pervious soils with thin overlying ground, due to possible blow-out.
In addition to shield driving, there are driving methods which partly employ additional auxiliary construction equipment when dealing with difficult soils. For example, tunnel construction methods use gunite, shotcrete, injection grouting, drawing off or lowering ground water or surface freezing, etc. All of these procedures are more or less conditionally applicable depending upon soil, water content, consistency, tunnel cross section, overlying ground and total costs.
When constructing a tunnel by soil freezing, bore holes are drilled parallel to the tunnel axis at certain intervals beyond the planned outer limits of the tunnel. Pipes are inserted into these bore holes, and a cooling agent is circulated in the pipes. The cooling agent removes heat from the ground, until a frozen soil and ice mass is formed around the planned tunnel cross section. The ice mass supports the surrounding ground during excavation and protects the tunnel space already driven against ground water and cave-in.
The costs of the latter process are relatively high for forming the ice mass and maintaining its operation. Other conventional processes have significant technical drawbacks. In tunnel structures near the surface which are to be constructed in vastly varying soils, and in cases where water seepage may occur, shielding by soil freezing can be employed only in conjunction with auxiliary construction equipment. The auxiliary construction equipment, as a rule, presents a considerable environmental burden and frequently is permitted only if there are no technical alternatives and if the particular project absolutely has to go forward.